1. Field of the Invention
The present invention relates to an automobile antenna system and particularly to an improved automobile antenna for effectively receiving and detecting broadcast waves at the vehicle body to feed detection signals to various receivers which are internally located in the vehicle body.
2. Description of the Prior Art
Antenna systems are essential for modern automobiles which positively receive various broadcast and communication waves to be supplied to various inboard receivers such as radios, televisions, telephones and the like. Such antenna systems also are very important as for transmission and reception of civil band waves to communicate an automobile with other sources of radio waves.
One of the conventional well-known antenna systems is in the form of a pole antenna protruded outwardly from the vehicle body, which has some preferred performances in its receiving characteristics, but which is disadvantageous in that the pole antenna may spoil the aesthetic appearance of the automobile.
Indeed, such a pole antenna is subject to damage and mischief and also tends to produce unpleasant noises when a vehicle runs at high speeds.
Recently, the number of frequency bands of broadcast or communication waves to be received at by automobiles has been increased. When one wishes to receive a plurality of frequency band waves, the corresponding number of antennas are required which may degrade the aesthetic appearance of an automobile. Some electrical interference may be raised between these antennas, leading to very a reduction of reception performance.
Some attempts have been made to provide an invisible antenna in place of the pole antenna. One such attempt is that an antenna wire is applied to the rearwindow glass of an automobile.
Another attempt has been made in which there is provided means for detecting surface currents induced on the vehicle body by broadcast waves. Although such a proposal appears to provide a positive and efficient means for receiving broadcast waves at an automobile, experiments show that it is unsuccessful.
Firstly, the unsuccessful means utilizing the surface currents induced on the vehicle body by broadcast waves results from the fact that the value of surface current is not large against expectation. Even when the surface currents induced in the roof panel of the vehicle body were utilized, one could not obtain sufficient levels of available detection output.
Secondly, the surface currents included noises in very large proportion. Such noises result mainly from the engine ignition system and battery charging regulator system and cannot be removed from the surface currents while the engine runs.
Still another attempt is disclosed in Japanese Patent Publication Sho No. 53-22418 in which an antenna system utilizing currents induced on a vehicle body by broadcast waves comprises an electrical insulation portion formed at the current concentration portion of the vehicle body and a sensor for directly detecting currents between the opposite ends of the electrical insulation portion. This antenna system exhibits a superior performance that practicable detection signals superior in SN ratio can be obtained. However, the antenna system includes a pickup structure which requires provision of a notch formed in part of the vehicle body. This cannot easily be accepted by manufacturers who produce automobiles in mass-production.
Japanese Utility Model Publication Sho No. 53-34826 discloses an antenna system comprising a pickup coil for detecting currents on the pillar structure of a vehicle body. This is advantageous in that the antenna can internally be mounted in the vehicle body. It is however impracticable that the pickup coil is located adjacent to the pillar in a direction perpendicular to the longitudinal axis thereof. Moreover, such arrangement cannot provide any practicable output from the antenna.